IEEE Solid-State Circuits Magazine - Summer 2015 - 26
Current
Output
MSB
I
I/2
I/4
I/8
VB
R
2R
4R
8R
-V
Figure 12: The quad-switch DAC building block with external thin film resistor network.
functions in successive approximation ADCs. These became standard
building blocks in many modular
and hybrid data converters. In fact,
the acronym SAR actually stands
for "successive-approximation register," and hence the term "SAR ADC."
Earlier converters utilizing the successive-approximation architecture
were simply referred to as sequential
coders, feedback coders, or feedback
subtractor coders.
Specifications and Testing
of Data Converters in the 1960s
There were no guidelines or industry standards for specifying and
testing the early data converter of
the 1960s, so the manufacturers did
the best they could based on their
knowledge of their products and
potential applications Bernard Gordon was active in this area along
with a few others, and [49]-[58] show
the evolution of the thinking relating to data converter specifications
and testing in the 1960s (see also
"Data Converter Architectures").
During the 1960s, it became clear
that dynamic specifications (SNR,
SFDR, THD, etc.) were important for
many applications, and in the early
1970s minicomputers such as the
PDP-11 enabled data analysis of ADC
outputs including FFTs and histograms. Prior to this time, dynamic
26
su m m E r 2 0 15
testing generally consisted of "backto-back" tests where the ADC output
drove a DAC, and analog test equipment was used to examine the reconstructed DAC output. The use of
computers to analyze the ADC output directly eliminated errors due to
the reconstruction DAC.
Data Converters of the 1970s
and the Beginnings of Integration
The year 1970 began one of the most
exciting decades in the history of
data converters. The ADC/DAC market was driven by many emerging
applications, including high resolution digital voltmeters, industrial
process control, digital video, military phased-array radar, medical
imaging, vector scan displays, and
raster scan displays. Most of these
systems had formerly utilized conventional analog signal processing techniques, and the increased
availability of low-cost computing
technology generated a desire to
take advantage of the increased performance and flexibility offered by
digital signal processing and analysis-and, of course, the need for
compatible data converters.
As a result, a number of companies entered the data converter field,
including Analog Devices, Analogic
Corporation (initially Epsco and later
Gordon Engineering), Burr Brown,
IEEE SOLID-STATE CIRCUITS MAGAZINE
Computer Labs, Datel, Hybrid Systems, ILC/Data Device Corporation,
Micronetworks, National Semiconductor, Teledyne Philbrick, and Zeltex.
IC building blocks, as well as complete IC data converters of the 1970s,
came from Analog Devices, Advanced
Micro Devices, Fairchild, Signetics,
Intersil, Micro Power Systems, Motorola, National Semiconductor, TRW (LSI
Division), and Precision Monolithics.
The data converters of the 1970s
made maximum use of all the technologies available: monolithic, modular, and hybrid, with the modular and
hybrid products typically offering
higher resolution and faster speed
than the existing monolithic parts.
Monolithic Bipolar DACs
of the 1970s
The earliest monolithic DACs were
made using bipolar process technology;
they included only the basic core of a
complete DAC-the array of switches
and resistors to set the weight of each
bit. Some examples were the Motorola
1408 and a later higher speed derivative, the DAC08, introduced in 1975 by
PMI (Precision Monolithics).
These converters were produced
by several manufacturers and were
available at low cost. However, they
required many additional external
components to be usable in a system design. These external components included several resistors, a
voltage reference, a latch, an output
op-amp, possibly a compensation
capacitor, and usually one or more
trimming potentiometers.
Converters like the 1408 and
DAC08 were limited to 8-bit accuracy
by the matching and tracking limitations of the diffused resistors. When
higher accuracy is required, lower
temperature coefficient resistors are
needed, and some means of post-fabrication adjustment is desirable.
Thin film resistors exhibit low
temperature coefficients and can be
trimmed with a laser-they are well
suited for use in data converters. By
the mid-1970s, Analog Devices had
developed considerable expertise,
not only in the deposition of thin
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